Document why some "type mismatches" exist

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -1619,6 +1619,9 @@ pub(in super::super) fn check_expr_lit(
             ast::LitKind::Int(_, ast::LitIntType::Unsuffixed) => {
                 let opt_ty = expected.to_option(self).and_then(|ty| match ty.kind() {
                     ty::Int(_) | ty::Uint(_) => Some(ty),
+                    // These exist to direct casts like `0x61 as char` to use
+                    // the right integer type to cast from, instead of falling back to
+                    // i32 due to no further constraints.
                     ty::Char => Some(tcx.types.u8),
                     ty::RawPtr(..) => Some(tcx.types.usize),
                     ty::FnDef(..) | ty::FnPtr(..) => Some(tcx.types.usize),